Methods of Hydraulic Compensation for Magnetically Biased Fluid Dynamic Bearing Motor

ABSTRACT

A fluid dynamic bearing motor including a stationary sleeve, a rotating shaft axially disposed through the sleeve, a journal gap between the shaft and the sleeve, the gap defined by first and second interfacial surfaces of the shaft and sleeve, at least one set of fluid dynamic grooves formed on the first interfacial surface of the journal gap, and at least one step defined on the second interfacial surface of the journal gap.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.10/602,471, filed Jun. 23, 2003, now U.S. Pat. No. 7,422,370, whichclaims the priority of U.S. Provisional Application No. 60/401,797,filed Aug. 6, 2002 by LeBlanc et al. (entitled “Hydraulic CompensationFor Magnetic Bias FDB Motor”), which is herein incorporated byreference.

FIELD OF THE INVENTION

The invention generally relates to fluid dynamic bearing motors and,more particularly, to magnetically biased fluid dynamic bearing motors.

BACKGROUND OF THE INVENTION

Disk drives are capable of storing large amounts of digital data in arelatively small area. Disk drives store information on one or morerecording media, which conventionally take the form of circular storagedisks (e.g. media) having a plurality of concentric circular recordingtracks. A typical disk drive has one or more disks for storinginformation. This information is written to and read from the disksusing read/write heads mounted on actuator arms that are moved fromtrack to track across the surfaces of the disks by an actuatormechanism.

Generally, the disks are mounted on a spindle that is turned by aspindle motor to pass the surfaces of the disks under the read/writeheads. The spindle motor generally includes a shaft mounted on a baseplate and a hub, to which the spindle is attached, having a sleeve intowhich the shaft is inserted. Permanent magnets attached to the hubinteract with a stator winding on the base plate to rotate the hubrelative to the shaft. In order to facilitate rotation, one or morebearings are usually disposed between the hub and the shaft.

Over the years, storage density has tended to increase, and the size ofthe storage system has tended to decrease. This trend has lead togreater precision and lower tolerance in the manufacturing and operatingof magnetic storage disks.

From the foregoing discussion, it can be seen that the bearing assemblythat supports the storage disk is of critical importance. One bearingdesign is a fluid dynamic bearing. In a fluid dynamic bearing, alubricating fluid such as air or liquid provides a bearing surfacebetween a fixed member of the housing and a rotating member of the diskhub. In addition to air, typical lubricants include gas, oil, or otherfluids. The relatively rotating members may comprise bearing surfacessuch as cones or spheres, or may alternately comprise fluid dynamicgrooves formed on the members themselves. Fluid dynamic bearings spreadthe bearing surface over a large surface area, as opposed to a ballbearing assembly, which comprises a series of point interfaces. Thisbearing surface distribution is desirable because the increased bearingsurface reduces wobble or run-out between the rotating and fixedmembers. Further, the use of fluid in the interface area imparts dampingeffects to the bearing, which helps to reduce non-repeat run-out. Thus,fluid dynamic bearings are an advantageous bearing system.

Many current fluid dynamic bearing designs employ a combination ofjournal and thrust bearings. Frequently, these designs include a shaftjournal bearing design having a thrust plate at an end thereof. Thejournal bearings typically include two grooved surfaces facing thejournal (either on the shaft or on the sleeve), the thrust platebearings typically include two grooved surfaces, one facing each of thegaps defined by the thrust plate and sleeve, and by the thrust plate andcounter plate. Net hydraulic pressure created by the journal bearingsestablishes a thrust force on the end of the shaft (i.e., toward thethrust plate bearings) that displaces the shaft axially; an opposingforce, generated, for example, by a magnetic bias force, is needed tostabilize the motor.

However, as the temperature fluctuates in the motor, the viscosity ofthe fluid in the bearings changes as well. While the magnetic bias forceremains constant regardless of temperature, the hydraulic pressure(thrust force) generated by the journal bearings varies with thechanging fluid viscosity. Thus, the opposing forces (thrust force vs.magnetic bias force) may not be of sufficient magnitudes to offset eachother, allowing the rotor to move axially as temperature changes.

Therefore, a need exists for a magnetically biased fluid dynamic bearingdesign that can compensate for changing temperature and fluid viscosityin the motor.

SUMMARY OF THE INVENTION

A fluid dynamic bearing motor comprising a stationary sleeve, a rotatingshaft axially disposed through the sleeve, a journal gap between theshaft and the sleeve, said gap defined by first and second interfacialsurfaces of the shaft and sleeve, at least one set of fluid dynamicgrooves formed on the first interfacial surface of the journal gap, andat least one step defined on the second interfacial surface of thejournal gap.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited embodiments of theinvention are attained and can be understood in detail, a moreparticular description of the invention, briefly summarized above, maybe had by reference to the embodiments thereof which are illustrated inthe appended drawings. It is to be noted, however, that the appendeddrawings illustrate only typical embodiments of this invention and aretherefore not to be considered limiting of its scope, for the inventionmay admit to other equally effective embodiments.

FIG. 1 depicts a plan view of one embodiment of a disk drive thatcomprises a motor in which the invention is used;

FIG. 2 depicts a side sectional view of a magnetically biased fluiddynamic bearing motor according to a first embodiment of the invention;

FIG. 2B depicts a groove pattern in accordance with the presentinvention.

FIG. 3 depicts a side sectional view of a magnetically biased fluiddynamic bearing motor according to a second embodiment of the invention;

FIG. 4 depicts a side sectional view of a magnetically biased fluiddynamic bearing motor according to a third embodiment of the invention;and

FIG. 5 depicts a side sectional view of a magnetically biased fluiddynamic bearing motor according to a fourth embodiment of the invention.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures.

DETAILED DESCRIPTION

FIG. 1 depicts a plan view of one embodiment of a disk drive 10 for usewith embodiments of the invention. Referring to FIG. 1, the disk drive10 includes a housing base 12 and a top cover plate 14. The housing base12 is combined with cover plate 14 to form a sealed environment toprotect the internal components from contamination by elements outsidethe sealed environment. The base and cover plate arrangement shown inFIG. 1 is well known in the industry; however, other arrangements of thehousing components have frequently been used, and aspects of theinvention are not limited by the particular configuration of the diskdrive housing.

Disk drive 10 further includes a disk pack 16 that is mounted on a hub202 (see FIG. 2) for rotation on a spindle motor (not shown) by a diskclamp 18. Disk pack 16 includes one or more of individual disks that aremounted for co-rotation about a central axis. Each disk surface has anassociated read/write head 20 that is mounted to the disk drive 10 forcommunicating with the disk surface. In the example shown in FIG. 1,read/write heads 20 are supported by flexures 22 that are in turnattached to head mounting arms 24 of an actuator 26. The actuator shownin FIG. 1 is of the type known as a rotary moving coil actuator andincludes a voice coil motor (VCM), shown generally at 28. Voice coilmotor 28 rotates actuator 26 with its attached read/write heads 20 abouta pivot shaft 30 to position read/write heads 20 over a desired datatrack along a path 32.

FIG. 2 is a sectional side view of a fluid dynamic bearing motor 200according to one embodiment of the present invention. The motor 200comprises a rotating assembly 205, a stationary assembly 203, and abearing assembly 207.

The rotating assembly 205 comprises a shaft 202 affixed at a first end221 to a hub 204 that supports at least one disk (not shown) forrotation. A second end 223 of the shaft 202 is distal from the first end221. The hub 204 additionally supports a magnet assembly 252 comprisinga back iron 211 with a magnet 209 affixed thereon. In one embodiment ofthe invention, the magnet assembly 252 is positioned on the insidecircumferential surface 254 of the hub 204.

The stationary assembly 203 comprises a sleeve 208 mounted on the base12. The sleeve 208 further comprises a bore 231 through which the shaft202 is disposed axially. A stator 210 mounted on the base 12 cooperateswith the magnet 209 in the hub 204 to induce rotation of the shaft 202and hub 204 relative to the sleeve 208. The stator 210 comprises aplurality of “teeth” 235 formed of a magnetic material such as steel,where each of the teeth 235 is wound with a winding or wire 237.

The bearing assembly 207 is formed in a journal (or gap) 217 definedbetween the facing surfaces of the inner diameter 215 of the sleeve 208and the outer diameter 219 of the shaft 202. A fluid 214 such as air,oil or gas is disposed between the shaft 202 and the sleeve 208. Thejournal 217 further comprises fluid dynamic grooves 300; an example isformed on one or both of the interfacial surfaces 215, 219 (in FIG. 2,the fluid dynamic grooves 300 are formed on the outer surface 219 of theshaft 202).

The fluid dynamic grooves 300 form a circumferential ring around aninterfacial journal surface 215, 219 and may comprise a V-shaped patternor a chevron, spiral or sinusoidal pattern or other pattern (not shown).The pattern, generates a pressure distribution across the bearingsurface that provides improved bearing rocking stiffness.

The fluid dynamic grooves 300 may be formed asymmetrically, where thelength of one leg of the pattern leading to the pattern's pressure apexis greater than the length of the leg on the other side of the pattern'sapex. When asymmetry of the pattern is created by legs with differentlengths, a net flow of fluid 214 is pumped toward the leg with theshorter length. As the hub 204 and shaft 202 rotate, a net hydraulicpressure is generated by the journal bearing grooves 300 toward thesecond end 223 of the shaft 202. Pressure is also generated as afunction of the size of the gap between the shaft 202 and sleeve 208 inthe areas of the grooves 300 (and depending on the size of the gap,symmetric grooves 300 may also be used, and the same effect achieved).This pressure exerts a positive thrust force on the second end 223 ofthe shaft 202 that displaces the shaft 202 axially.

One way to balance the asymmetry pressure acting on the shaft 202 is tooffset the magnet 209 and stator 210 relative to each other to create amagnetic bias force that biases the hub 204 downward and stabilizes themotor 200. As illustrated in FIG. 2, the center lines of the magnet 209and stator 210 are separate by a vertical distance of d. This method hasgenerally proven to be effective; however, temperature changes in themotor may limit or hinder the ability of the magnetic force to bias thehub 204. This is because the viscosity of the fluid 214 varies withchanges in temperature, which means that the journal asymmetry pressureis not constant, but rather may be a function of temperature. Therefore,because the magnetic force can not be varied accordingly to address andcounter the changes in journal asymmetry pressure, temperaturevariations will cause the shaft 202 and hub 204 to move axially.

One solution to this problem would be to use the axial shaftdisplacement to change the length of the asymmetry created by thejournal bearings 300. However, the axial displacement required toeffectively counter the pressure changes would likely be too great to bepractically incorporated. In the embodiment illustrated in FIG. 2,journal asymmetry pressure fluctuations are countered by changing thegap width between the shaft 202 and sleeve 208 in the asymmetricportions of the journal bearings 300A. This is accomplished by creatinga step 260 on the journal surface 215, 219 that is opposite theasymmetric grooves 300. In FIG. 2, the step 260 is located on the innerdiameter 215 of the sleeve 208, opposite the journal bearing grooves 300on the shaft 202. The step 260 is also offset axially from the grooves300, so that when the motor 200 is at rest, the gap separating thegrooved portion of the shaft 202 from the sleeve 208 is a standard widthw₁. Although there is a small axial overlap of the step and grooves, theapex 304 of the grooves 300 is generally adjacent a gap of standardwidth w₁. Thus, as the shaft 202 moves downward, the grooves 300 movecloser axially to the step 260, and the width of the gap separating theupper portion of the grooved area (i.e., mostly the upper leg of thegroove pattern) of the shaft 202 from the sleeve 208 shrinks to a gapw₂. As the gap in this region tightens, more pressure is built up at thebottom of the shaft 202, and the pressure pushes the shaft back up.Furthermore, this design provides additional stiffness (pressure changevs. axial movement of shaft) to the motor, reducing or eliminating theneed for either a thrust plate with grooves or for a tight thrust gap,which draws constant power.

Typical fluid dynamic bearing motors have journal bearing gaps on theorder of five microns or less, and changes to the gap must be controlledto a fraction of that number. Therefore, processes used to create thesteps 260 and must be very precise. Steps 260 may be created either byremoving material from the shaft 202 or sleeve 208 (e.g., by processesincluding, but not limited to, turning, grinding, electrochemicalmachining, or electrical discharge machining), or by adding material tothe surfaces 202, 208 (e.g., by processes including, but not limited to,plating, coating, or sputtering). For example, Diamond Like Coating(DLC) may be sputtered onto the appropriate area.

FIG. 3 illustrates a second embodiment of the present invention. Themotor 400 is configured similarly to the motor 200 in FIG. 2. However,in FIG. 3, the step 460 is formed on the outer diameter 419 of the shaft402, rather than on the inner diameter 415 of the sleeve 408. The step460 operates in the same manner as the step 260 in FIG. 2, to narrow thebearing gap and thus counter hydraulic pressure variations.

A third embodiment of the invention is illustrated in FIG. 4. The motor500 is similar to the motors 200 and 400 illustrated in FIGS. 2 and 3.However, the journal 517 comprises two steps 560A, 560B located acrossthe journal from each set of asymmetry grooves 300. Although the steps560A, 560B are depicted as formed on the outer diameter 519 of the shaft502, it will be appreciated that the steps 560A, 560B may also be formedon the inner diameter 515 of the sleeve 508, as the step 260 is locatedin FIG. 2.

FIG. 5 illustrates a fourth embodiment of the invention. The motor 600is similar to the motor 500 in FIG. 4 and uses the same principle of adouble step. However, the journal steps 660A, 660B are made larger thanin the previous embodiments so that they interact with larger areas ofthe bearing grooves 300. The grooves 300 comprise two legs: L₁ thatpumps upward toward the apex, and L₂ that pumps downward toward theapex. When the motor 600 is at rest, a majority of each set of grooves300 is adjacent a gap of standard width w₁, while the edges of thegrooves are just bordered by a narrower gap w₂ created by the steps660A, 660B. However, as the shaft 602 moves downward axially, the gapnarrows over a larger portion of the grooves 300, and narrows completelyover the upper legs L₂, to a width of w₂. The narrower gap over theupper legs L₂ takes away the downward pumping of these legs, and alsodiminishes the upward pumping of the lower legs L₁. Thus each individualset of bearing grooves 300 is affected to a larger degree over both legsL₁, L₂, unlike the previous embodiments that affected the gaps adjacentsmaller portions of the bearing grooves, and more particularly narrowedthe gaps mostly adjacent the upper legs of the grooves.

Thus the present invention represents a significant advancement in thefiled of fluid dynamic bearing motor design. A magnetically biased fluiddynamic bearing motor is provided in which axial movement of the shaftand hub is limited despite temperature-induced pressure fluctuations inthe journal. The design also provides improved stiffness to the motor,reducing or eliminating the need for thrust plate bearing grooves ortight thrust gaps. In addition to the thermal compensation effects, themotor doesn't need a thrust bearing as the thrust is created by thejournal asymmetry. This asymmetry is created by asymmetric bearinggrooves in the journal bearing (as described above) and/or by the properlocation of the step or steps 260 relative to the groove pattern in thejournal bearing. Positioning of the step or steps 260 alters thepressure profile in the journal bearing and thus the pressure on thebottom of the shaft to support the shaft for rotation over the base. Inother words, when the reduced gap width provided by the step 260 is overthe grooves, the effect is asymmetry whether or not the groove patternitself being asymmetric. Also, either the end surface of the shaft (e.g.280, FIG. 2) or the facing surface of the base 12 or counterplate 282may be grooved to provide a quicker take-off when the motor shaft 202spins up. The groove pattern would typically be designed to pump towardthe center of the shaft. However, even in this instance the thruststiffness is primarily created by journal bearing asymmetry, establishedin whole or in part by the step facing the journal groove pattern.

While the foregoing is directed to embodiments of the invention, otherand further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof isdetermined by the claims that follow.

1. A method of operating a motor having a fluid dynamic bearing formedbetween a fixed sleeve and a rotatable shaft disposed in the sleeve, andthe bearing having a fluid with a variable viscosity disposed therein,the method comprising: generating a thrusting force for lifting therotatable shaft from fluid pressure created, at least in part, byinteraction between a surface of one of the shaft and sleeve and pumpinggrooves disposed on the other of the shaft and the sleeve, whereby thethrusting force produces an amount of lift determined at least in partby the fluid viscosity; and dynamically adjusting the thrusting force toregulate the amount of lift by varying a size of a gap formed betweenthe surface and the pumping grooves.
 2. The method of claim 1, whereinthe size of the gap is varied by relative axial movement of the shaftand the sleeve, wherein the surface includes one or more steps proudfrom a remainder of the surface, such that as the pumping grooves becomemore aligned with the steps, the gap decreases, causing the fluidpressure to increase, and produce more thrusting force.
 3. The method ofclaim 2, wherein the pumping grooves are disposed on an outer diameterof the shaft, and the surface comprises an inner diameter of the sleeve.4. The method of claim 2, wherein the pumping grooves are disposed on aninner diameter of the sleeve, and the surface comprises an outerdiameter of the shaft.
 5. The method of claim 1, wherein the pumpinggrooves are asymmetric to establish pumping pressure toward an end ofthe shaft.
 6. The method of claim 1, wherein generating the thrustingforce for lifting the rotatable shaft further comprises a thrustingforce generated by pumping grooves on at least one of a counterplate anda facing surface of the shaft.
 7. A motor having a fluid dynamic bearingformed in a gap disposed between a fixed sleeve and a rotatable shaftdisposed in the sleeve, lubricating fluid disposed in the gap, the motorformed by a method comprising: providing a base under the shaft;defining one or more steps proud from a surface of one of the fixedsleeve and the rotatable shaft; and defining pumping grooves on asurface of the other of the fixed sleeve and the rotatable shaft, thepumping grooves positioned relative to the one or more steps toestablish, at least in part, during operation, an asymmetric fluidpressure profile that supports the shaft for rotation over the base, andresponds to effects from fluid viscosity changes that affect theasymmetric fluid pressure profile by an relative axial movement betweenthe grooves and the one or more steps, thereby countering the effectsfrom fluid viscosity changes to the asymmetric fluid pressure profile.8. The method of claim 7, wherein the pumping grooves are asymmetric toestablish pumping pressure toward an end of the shaft.
 9. The method ofclaim 7, wherein defining the one or more steps comprises addingmaterial to the surface of one of the fixed sleeve and the rotatableshaft by at least one of the following methods: plating, coating, andsputtering.
 10. The method of claim 9, wherein the material is added bysputtering a Diamond Like Coating upon the surface of one of the fixedsleeve and the rotatable shaft.
 11. The method of claim 7, whereindefining the one or more steps comprises removing material from thesurface of one of the fixed sleeve and the rotatable shaft by at leastone of the following methods: turning, grinding, electrochemicalmachining, and electrical discharge machining.
 12. The method of claim7, wherein the pumping grooves are defined on an inner diameter of thesleeve, and the surface opposite the pumping grooves comprises an outerdiameter of the shaft.
 13. The method of claim 7, wherein the pumpinggrooves are defined on an outer diameter of the shaft, and the surfaceopposite the pumping grooves comprises an inner diameter of the sleeve.14. The method of claim 7, wherein the pumping grooves are asymmetric toestablish pumping pressure toward an end of the shaft proximate thebase.
 15. The method of claim 7 further comprising defining pumpinggrooves on at least one of the base under the shaft and a surface of theshaft opposite the base to aid in establishing the pressure profile thatsupports the shaft for rotation over the base.
 16. A method forcountering effects from fluctuating fluid viscosity of fluid disposed ina gap of a fluid dynamic bearing comprising: moving a first surfaceaxially relative to a second surface during operation at least in partby the effects from fluctuating fluid viscosity; generating hydraulicforce by interaction between at least one set of fluid dynamic grooveson the first surface and at least one step extending from the secondsurface; and countering the axial movement of the first surface relativeto the second surface by, at least in part, the hydraulic force changingto compensate for the effects from fluctuating fluid viscosity thatwould otherwise cause increased axial movement of the first surfacerelative to the second surface.
 17. The method of claim 16, wherein thefluid dynamic grooves are defined on an inner diameter of a sleeve, andthe second surface comprises an outer diameter of the shaft.
 18. Themethod of claim 16, wherein the fluid dynamic grooves are defined on anouter diameter of a shaft, and the second surface comprises an innerdiameter of the sleeve.
 19. The method of claim 16, wherein the pumpinggrooves are asymmetric to establish pumping pressure toward an end ofthe shaft adjacent the base.
 20. The method of claim 16, wherein themoving comprises operating a motor, in which the fluid dynamic bearingis a part, the operating resulting in changing fluid viscosity